Video Encoding Apparatus, Video Decoding Apparatus, and Video Decoding Method for Performing Intra-Prediction Based on Directionality of Neighboring Block

ABSTRACT

Provided are methods and apparatuses for improving compression efficiency in directional intra-prediction. A video encoding apparatus, which does not need to record intra mode information, includes a mode selector that selects one from among a plurality of intra modes on the basis of a directionality of at least one neighboring block that has already been reconstructed before a current block is reconstructed, an intra predictor that obtains a prediction block of the current block from the at least one neighboring block according to the directionality of the selected mode, and obtains a residual block by subtracting the prediction block from the current block, and a unit for encoding the obtained residual block.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 2009-0134017, filed Dec. 20, 2009, the disclosure ofwhich is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

Example embodiments of the present invention relates to a videocompression method and, more particularly, to a method and apparatus forimproving compression efficiency in directional intra-prediction.

2. Related Art

With the development of telecommunication technologies including theInternet, video communication is increasing in addition to textcommunication and voice communication. It is insufficient to satisfyvarious desires of consumers with existing text-based communication.Thus, multimedia service capable of covering various types ofinformation such as text, image, music, etc. is increasing. Multimediadata requires a high-capacity storage medium due to its enormous volume,and a wide bandwidth when transmitted. Thus, to transmit the multimediadata including text, video, and audio, it is essential to use acompression coding technique.

A fundamental principle of compressing data is based on a process ofeliminating the redundancy from data. The data can be compressed byeliminating spatial redundancy referring to repetition of the same coloror object in an image, temporal redundancy referring to little ornothing of variation between neighboring frames in a moving pictureframe or successive repetition of same sound in the audio, orpsycho-visual redundancy referring to dullness of human vision andsensation to high frequencies.

As a method of compressing this moving picture, a growing interest inH.264 or advanced video coding (AVC) that further improves compressionefficiency compared to Moving Picture Experts Group-4 (MPEG-4) hasrecently been taken. As a scheme for improving the compressionefficiency, H.264 employs directional intra-prediction (hereinafter,shortened simply to “intra-prediction”) to eliminate spatial redundancywithin a frame.

The intra-prediction refers to a method of coping one sub-block in adesignated direction using neighboring pixels in upward and leftwarddirections, predicting values of current sub-blocks, and encoding onlythe differences between the copied values and the predicted value of thesub-blocks. By contrast, inter-prediction or temporal prediction refersto a method of performing prediction with reference to the area 40 of aframe 20 having a difference of time, as shown in FIG. 1. Thisintra-prediction is complementary to the inter-prediction. In otherwords, one of the two prediction methods that is favorable for the imageto be encoded is selectively used.

In the intra-prediction technique complying with the existing H.264standard, a prediction block is generated from a current block on thebasis of another block having a previous encoding sequence. A valuesubtracting the prediction block from the current block is encoded. Interms of a luminance component, the prediction block is generated inunits of 4×4 blocks or 16×16 macroblocks. Nine prediction modes can beselected for each 4×4 block, whereas four prediction modes can beselected for each 16×16 macroblock. A video encoder based on H.264selects one from among the prediction modes with respect to each block.In the selected prediction mode, a difference between the current blockand the prediction block is minimized.

As the prediction mode for the 4×4 block, nine prediction modesincluding a total of eight modes (modes 0, 1, and 3 through 8) havingdirectionality and a direct current (DC) mode (mode 2) using a meanvalue of eight neighboring pixels are used in H.264, as shown in FIG. 2.

FIG. 3 shows an example of labeling for explaining the nine predictionmodels. In this case, a prediction block (areas including a through p)is generated from the current block using samples A through M that arepreviously decoded. Here, if E, F, G and H cannot be previously decoded,D is copied at their positions, so that E, F, G and H can be virtuallygenerated.

The nine prediction modes will be described in detail with reference toFIG. 4. In the case of mode 0, pixels of the prediction block areestimated by extrapolation in a vertical direction using upper samplesA, B, C and D. In the case of mode 1, the pixels of the prediction blockare estimated by extrapolation in a horizontal direction using leftsamples I, J, K and L. Further, in the case of mode 2, the pixels of theprediction block are identically contrapositioned on the average of theupper samples A, B, C and D and the left samples I, J, K and L.

Further, in the case of mode 3, the pixels of the prediction block areestimated by interpolation at an angle of 45° in an upper-right tolower-left (diagonal down-left) direction. In the case of mode 4, thepixels of the prediction block are estimated by extrapolation at anangle of 45° in an upper-left to lower-right (diagonal down-right)direction. Further, in the case of mode 5, the pixels of the predictionblock are estimated by extrapolation at an angle of about 26.6°(width/height=1/2) in a vertical-right direction.

In addition, in the case of mode 6, the pixels of the prediction blockare estimated by extrapolation at an angle of about 26.6° in ahorizontal-down direction. In the case of mode 7, the pixels of theprediction block are estimated by extrapolation at an angle of about26.6° in a vertical-left direction. Finally, in the case of mode 8, thepixels of the prediction block are estimated by interpolation at anangle of about 26.6° in a horizontal-up direction.

Arrows of FIG. 4 indicate prediction directions in each mode. Thesamples of the prediction block in modes 3 through 8 can be generatedfrom a weighted average of previously-decoded reference samples Athrough M. For example, in the case of mode 4, the sample d located atthe upper right edge of the prediction block can be estimated as inEquation 1 below. Here, round( ) is the integer round-off function.

d=round (B/4+C/2+D/4)  [Equation 1]

Meanwhile, there are four modes 0, 1, 2, and 3 in the 16×16 predictionmodel for a luminance component. In the case of mode 0, the pixels ofthe prediction block are estimated from upper samples by extrapolation.In the case of mode 1, the pixels of the prediction block are estimatedfrom left samples by extrapolation. Further, in the case of mode 2, thepixels of the prediction block are calculated by averaging the uppersamples and left samples. Finally, in the case of mode 3, the pixels ofthe prediction block use a linear “plane” function that is fitted to theupper and left samples. This mode works well in areas ofsmoothly-varying luminance.

In this manner, the video encoder based on the H.264 standard performsintra-prediction using a total of nine modes including eight modeshaving directionality (hereinafter, referred to as “directional modes”)and one DC mode. Intra mode information representing one of the ninemodes obtained in this way is transmitted to a video decoder. The videodecoder obtains a prediction block from a current block in the samemethod as the video encoder on the basis of the intra mode information,and reconstructs the current block from the obtained prediction block.

SUMMARY

However, since this intra mode information is given to each 4×4 block,it may act as an overhead increasing the size of a coded bitstream. Thismeans that the intra mode information is required per 16 pixels, andthus is by no means a small amount compared to a result of encoding aresidual.

Example embodiments of the present invention is directed to provide avideo encoding method and apparatus, and a video decoding method andapparatus, for enhancing efficiency of intra-prediction in video coding.

It is to be understood that technical problems to be solved by thepresent invention are not limited to the aforementioned technicalproblems, and other technical problems which are not mentioned will beapparent from the following description to persons with ordinary skillin the art to which the present invention pertains.

In some example embodiments, there is provided a video encodingapparatus, which does not need to record intra mode information. Thevideo encoding apparatus includes: a mode selector configured to selectone from among a plurality of intra modes on the basis of directionalityof at least one neighboring block, the at least one neighboring blockhaving already been reconstructed before the current block isreconstructed; an intra predictor configured to obtain a predictionblock of the current block from the at least one neighboring blockaccording to the directionality corresponding to the selected mode, andconfigured to obtain a residual block by subtracting the predictionblock from the current block; and means for encoding the obtainedresidual block.

In other example embodiments, there is provided a video decodingapparatus, which does not need to receive intra mode information. Thevideo decoding apparatus includes means for reconstructing a residualsignal of a current block from an input bitstream; a mode selectorconfigured to select one from among a plurality of intra modes on thebasis of directionality of at least one neighboring block, the at leastone neighboring block having already been reconstructed before thecurrent block is reconstructed; and an intra reconstructor configured toobtain a prediction block of the current block from the at least oneneighboring block according to a directionality of the selected mode,and configured to reconstruct the current block by adding the residualsignal of the current block and the prediction block.

In still other example embodiments, there is provided a video decodingmethod, which includes reconstructing a residual signal of a currentblock from an input bitstream; selecting one from among a plurality ofintra modes; and performing an intra-prediction according to adirectionality of the selected intra mode to reconstruct the currentblock. The plurality of intra modes may use 34 modes for an 8×8 block.The 34 modes may have a maximum of 34 directions, each of which may bean arbitrary direction with respect to on the basis of an arbitrarypixel within the current block, the arbitrary direction may have a slopedefined as dy in a vertical direction over dx in a horizontal direction,wherein dx and dy may be natural numbers. The number of the plurality ofintra modes may be different depending on a block size. The plurality ofintra modes may include at least one of 9 intra modes for a 4×4 block, 9intra modes for an 8×8 block, 34 intra modes for a 16×16 block, 34 intramodes for a 32×32 block, 5 intra modes for a 64×64 block, and 5 intramodes for a 128×128 block. The selecting of one from among a pluralityof intra modes may include selecting one from among the plurality ofintra modes on the basis of a directionality of at least one neighboringblock that has already been reconstructed before the current block isreconstructed. The performing an intra-prediction according to adirectionality of the selected intra mode to reconstruct the currentblock may includes: obtaining a prediction block of the current blockfrom the at least one neighboring block according to the directionalityof the selected mode; and reconstructing the current block by adding theresidual signal of the current block and the prediction block the atleast one neighboring block may include at least one of a upper block, aleft block, a upper left block, and a upper right block with respect tothe current block.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of exampleembodiments of the present invention will become more apparent to thoseof ordinary skill in the art by describing in detail example embodimentsthereof with reference to the attached drawings, in which:

FIG. 1 shows comparison of intra-prediction with inter-prediction;

FIG. 2 shows directionality of a conventional intra-prediction mode;

FIG. 3 shows an example of labeling for explaining the intra-predictionmode of FIG. 2;

FIG. 4 shows details of the intra-prediction mode of FIG. 2;

FIG. 5 is a block diagram showing the construction of a video encoderaccording to an example embodiment of the present invention;

FIG. 6 is a block diagram showing the construction of a video decoderaccording to an example embodiment of the present invention;

FIGS. 7A through 7H show a first detailed example embodiment ofgenerating a prediction block of intra-prediction of the presentinvention;

FIG. 8 shows a second detailed example embodiment of generating aprediction block of intra-prediction of the present invention; and

FIG. 9 shows the structure of a bitstream generated from a video encoderaccording to an example embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Hereinafter, example embodiments of the present invention will bedescribed in detail. The above and other objects, features andadvantages of example embodiments of the present invention will becomemore apparent to those of ordinary skill in the art by describing indetail example embodiments thereof with reference to the attacheddrawings. However, example embodiments of the present invention is notlimited to the embodiments disclosed below, but can be implemented invarious forms. These embodiments are rather provided so that thisdisclosure will be through and complete, and will fully convey the scopeof the invention to those skilled in the art. The scope of the inventionis merely defined by the appended claims and its equivalents. In thefollowing detailed description, the same reference numeral will be usedfor the same component or components regardless of the figures in orderto facilitate understanding of example embodiments of the invention.

There are two types of data, which are to be encoded afterintra-prediction. One of them is texture data of a residual blockgenerated by a difference between a block predicted from neighboringblocks and a current block, and the other is intra mode informationapplied to each block. An intra-prediction method proposed by exampleembodiments of the present invention is intended to provide technologyfor performing the intra-prediction without the intra mode information.In example embodiments of the present invention, the term “block” willbe used as concept covering a macroblock or a sub-block (8×8 block or4×4 block) having a smaller size than the macroblock.

FIG. 5 is a block diagram showing the construction of a video encoder200 according to an example embodiment of the present invention.

The video encoder 200 may include a block divider 205, a mode selector210, an intra-predictor 215, a spatial transformer 220, a quantizer 230,an entropy encoder 240, and a closed-loop decoder 250.

The block divider 205 divides an input image into blocks of apredetermined size. The size of each block may be 4×4, 8×8, or 16×16according to whether the input image is a chrominance signal or aluminance signal, or according to a different condition. In exampleembodiments of the present invention, the description will be made onthe assumption that a current block to be encoded is a block having a4×4 size and has a total of nine intra modes (mode 0 through mode 8).Further, the divided blocks may be formed by symmetrical partition in asquare shape of 4×4, 8×8, or 16×16, or by asymmetrical partition invarious geometrical shapes such as a rectangular shape of 4×16, 12×16,24×32, 8×32, 16×64, or 48×64, an “L” shape, a triangular shape, and soon.

The mode selector 210 according to an example embodiment of the presentinvention selects a mode suitable for the current block from among aplurality of intra modes using “directionality” of neighboring blocks ofthe current block. The neighboring blocks have already beenreconstructed before the current block is reconstructed. This method ofselecting the mode is different from a method of calculating a cost ofeach mode for the current block on the basis of a rate-distortion costas in the conventional H.264 and then selecting the mode that minimizesthis cost.

In detail, since the video decoder cannot directly use information aboutthe current block to be reconstructed unlike the video encoder,information that can be used in common between the video encoder and thevide decoder should be used. This is because, unlike the conventionalH.264, the intra mode information is not encoded in an exampleembodiment of the present invention. That is, in an example embodimentof the present invention, unlike the conventional H.264, the encoderdoes not transmit the intra mode information, and the decoder can obtainan optimal intra mode, in which a cost function is minimized on thebasis of the directionality of the neighboring blocks that have alreadybeen reconstructed before the current block is reconstructed, for thecurrent block, and thereby perform the intra-prediction. Alternatively,in another example embodiment of the present invention, the encoder mayobtain an optimal intra mode, in which a cost function is minimized onthe basis of the directionality of the neighboring blocks that havealready been reconstructed, for the current block, and then may transmitthe obtained intra mode information to the decoder.

In detail, an example embodiment of the present invention employs amethod of selecting the intra mode of the current block on the basis ofthe directionality of at least one of the neighboring blocks that havealready been reconstructed. Here, the directionality of the neighboringblock(s) may refer to, for instance, an optimal direction, which isestimated from the neighboring blocks, among the nine directions as inH.264 (rather than a method of directly calculating the cost of thecurrent block unlike H.264). That is, the intra-prediction according toan example embodiment of the present invention selects the intra mode ofthe current block on the basis of the directionality of the neighboringblocks that have already been reconstructed. By contrast, the existingH.264 is different from example embodiments of the present invention inthat it employs a method of performing the intra-prediction on thecurrent block using the nine intra modes and selecting the intra modehaving a minimum cost, rather than the method of selecting the intramode of the current block on the basis of the directionality of theneighboring blocks that have already been reconstructed as in exampleembodiments of the present invention.

The image associated with the neighboring blocks is information that canbe used in common in the video encoder as well as the video decoder.However, to be completely matched with the video decoder, theneighboring blocks are preferably obtained from an image reconstructedby closed-loop decoding, i.e. encoding and decoding, rather thanobtained from the original input image.

The closed-loop decoder 250 serves to provide this image, which isreconstructed for the neighboring blocks, to the mode selector 210. Theclosed-loop decoder 250 obtains an image reconstructed by performingdequantization and inverse spatial transform on the result quantized bythe quantizer 230 again, and then inversely performing original coding(inter coding, intra coding, or the like) on the result obtained by theinverse spatial transform. Note that, because the current block ispredicted by the intra-coding, its neighboring blocks are all subjectedto the intra coding, and then the image does not need to bereconstructed. Thus, it does not matter that the neighboring blocks areobtained from the image, which is reconstructed after being encoded byinter coding, intra coding, or other coding.

It has been described that the mode selector 210 selects the modesuitable for the current block using the directionality of theneighboring blocks. This description will be made below in greaterdetail with reference to FIG. 7.

The intra-predictor 215 obtains a prediction block of the current blockaccording to the intra mode selected by the mode selector 210, andsubtracts the obtained prediction block from the current block, therebyobtaining a residual block (residual signal). Once the intra mode (e.g.one of the nine modes of FIG. 4) is determined by the mode selector 210,the prediction block can be obtained according to the determined intramode using the same method as in the conventional H.264 of FIG. 4 (inthe case of the luminance signal, the nine intra modes are used for the4×4 block, and the four intra modes are used for the 16×16 block; in thecase of the chrominance signal, the four intra modes are used for the8×8 block). Example embodiment of the present invention is not limitedto this method. Thus, the prediction block may be obtained by anothercodec than H.264.

Here, as an example of the other codec than H.264, in the case of the8×8 block, the intra-prediction may be performed using 34 modes. Here,the 34 modes may represent a maximum of 34 directions, each of which isan arbitrary direction with respect to an arbitrary pixel within thecurrent block. The arbitrary direction has a slope defined as dy in avertical direction over dx in a horizontal direction (where dx and dyare natural numbers).

As another example of the other codec than H.264, a different number ofintra modes may be used according to a block size. For example, 9 intramodes may be used for a 4×4 block, 9 intra modes may be used for an 8×8block, 34 intra modes may be used for a 16×16 block, 34 intra modes maybe used for a 32×32 block, 5 intra modes may be used for a 64×64 block,and 5 intra modes may be used for a 128×128 block.

The residual block obtained by the intra predictor 215 is provided tothe spatial transformer 220 in order to perform spatial transform suchas discrete cosine transform (DCT).

Although not shown in FIG. 5, the video encoder 200 may further includean inter predictor. The inter predictor performs inter-prediction on theblocks divided by the block divider 205. That is, the inter predictorcan estimate motion for the divided blocks in units of blocks andgenerate motion vectors. Here, the divided blocks used for the motionestimation may vary in size. The divided blocks may be formed bysymmetrical partition in a square shape of 4×4, 8×8, or 16×16, or byasymmetrical partition in various geometrical shapes such as arectangular shape of 4×16, 12×16, 24×32, 8×32, 16×64, or 48×64, an “L”shape, a triangular shape, and so on. The inter predictor may performmotion compensation using the generated motion vector and a referencepicture, and generate a prediction block.

Although not shown in FIG. 5, the video encoder 200 may further includea subtractor, which subtracts the prediction block provided by the interpredictor from the current block and generates a residual value. Theinter-prediction and the intra-prediction may be selectively used foreach block.

The spatial transformer 220 removes spatial redundancy from the residualblock provided by the intra predictor 215 using spatial transform. Asthis spatial transform, discrete cosine transform (DCT) or wavelettransform may be used. A coefficient obtained by the spatial transformis referred to as a transform coefficient. Particularly, when the DCT isused as the spatial transform, the transform coefficient is referred toas a DCT coefficient.

The quantizer 230 quantizes the transform coefficient obtained by thespatial transformer 220. The quantizing process refers to a process ofdividing the transform coefficient expressed by an arbitrary real numbervalue by a constant section, representing the divided transformcoefficient by a discrete value, and matching it with a predeterminedindex. Particularly, when the wavelet transform is used as the spatialtransform, embedded quantization may be used as the quantization.

The entropy encoder 240 losslessly encodes the transform coefficientquantized by the quantizer 230, the difference value which is theresidual value, between the prediction block provided by the interpredictor and the current block, or the direction difference provided bythe intra predictor 215, and generates a bitstream. As the losslesscoding, arithmetic coding, variable length coding, Huffman coding, etc.may be used.

FIG. 6 is a block diagram showing the construction of a video decoder500 according to an example embodiment of the present invention.

The video decoder 500 may include an entropy decoder 510, a dequantizer520, an inverse spatial transformer 530, a mode selector 540, an intrareconstructor 550, and an inter reconstructor 560.

The entropy decoder 510 performs lossless decoding that is reverse toentropy encoding, and extracts motion data (data relevant to motionvector and reference frame information) in the inter-prediction. Theextracted texture data is provided as a quantization coefficient to thedequantizer 520, and the motion data is provided to the interreconstructor 560.

The dequantizer 520 dequantizes the quantization coefficient transmittedfrom the entropy decoder 510. The dequantizing process refers to aprocess of searching for the transform coefficient matched with thevalue expressed and transmitted by the predetermined index at the stageof the video encoder 200. A table representing a matching relationbetween the indices and the transform coefficients may be transmittedfrom the stage of the video encoder 200, and be previously set byarrangement between the encoder and the decoder.

The inverse spatial transformer 530 inversely performs spatialtransform, and transforms a transform coefficient (frequency domain)provided by the dequantization into a residual signal in a spatialdomain. For example, when the spatial transform is performed on thebasis of the wavelet transform at the stage of the video encoder, theinverse spatial transformer 530 will perform inverse wavelet transform.When the spatial transform is performed on the basis of the DCT at thestage of the video encoder, the inverse spatial transformer 530 willperform inverse DCT.

The mode selector 540 selects a mode suitable for the current block fromamong the nine intra modes using the directionality of the neighboringblocks of the current block which have already been reconstructed. Ofcourse, the neighboring blocks may be blocks that are reconstructed bythe intra reconstructor 550 or that are to be reconstructed by the interreconstructor 560. In this manner, the method of determining the intramode at the mode selector 550 is identical to that of selecting theintra mode at the mode selector 210 of the video encoder 200. Thus,according to an example embodiment of the present invention, the videodecoder 500 need not receive information about the intra mode from thevideo encoder 200. It has been described that the mode selector 540selects the mode suitable for the current block using the directionalityof the neighboring blocks. This description will be made below ingreater detail with reference to FIG. 7.

The intra reconstructor 550 obtains a prediction block of the currentblock according to the intra mode selected by the mode selector 540,adds the prediction block and the residual signal of the current blockprovided from the inverse spatial transformer 530, and reconstructs thecurrent block. Once the intra mode is determined by the mode selector540, the prediction block can be obtained according to the determinedintra mode (e.g., using the same method as in the conventional H.264 ofFIG. 4 or another codec than H.264 described above).

Meanwhile, to reconstruct the current block from the inter-prediction,the inter reconstructor 560 is used. This is because, although thecurrent block will be reconstructed by the intra-prediction, it does notmatter by which method its neighboring blocks are decoded.

The inter reconstructor 560 performs motion compensation on thepreviously reconstructed frame using the motion data provided from theentropy decoder 510, and generates a motion-compensated frame. Ofcourse, this motion compensating process is applied only when thecurrent block to be reconstructed is encoded through theinter-prediction process at the stage of the video encoder. Further,when the residual block reconstructed by the inverse spatial transformer530 is generated by the inter-prediction, the inter reconstructor 560adds the residual block and a domain corresponding to it within themotion-compensated frame, and thereby reconstructs the current block.

As described above, when the blocks reconstructed by the intrareconstructor 550 and the inter reconstructor 560 are assembled, thereconstructed frame can be finally completed.

Up to now, each component of FIGS. 5 and 6 may refer to software orhardware such as a field-programmable gate array (FPGA) or anapplication-specific integrated circuit (ASIC). However, thesecomponents are not limited to the software or hardware, and thus may beconstructed so as to be present in an addressable storage medium or toenable one or more processors. Functions provided within the componentscan be implemented by subdivided components or one component thatcombines a plurality of components to perform a specific function.

FIGS. 7A through 7H show a detailed method of a mode selector 210 or 540selecting a mode suitable for a current block from among a plurality ofintra modes using directionality of neighboring blocks of the currentblock which have already been reconstructed in accordance with anexample embodiment of the present invention. The neighboring blocks mayinclude blocks that have already been reconstructed. For example, theneighboring blocks may include an upper block, a left block, an upperleft block, and an upper right block, all of which have already beenreconstructed with respect to the current block by the same method asH.264.

First, the mode selector 210 or 540 calculates a cost C_(i) with respectto a specific directionality i by means of a sum (or an average) ofdeviations according to the directionalities using Equation 2 below.

$\begin{matrix}{C_{i} = {\sum\limits_{k = 0}^{n - 1}\; D_{ik}}} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

In Equation 2, n is the number of pixel pairs k for calculating thedeviation, i is the intra mode number, and D_(ik) is the deviationbetween the pixel pairs k corresponding to the specific directionalityi. The symbol n may be selected by another number depending on anembodiment. In example embodiments of the present invention, thedescription will be made on the assumption that n is set to 12.

FIG. 7A shows an example of calculating a cost of a vertical direction.The cost C₀ of the vertical direction is obtained by selecting 12 pixelpairs in the vertical direction from among adjacent pixels of theneighboring blocks of the current block, and calculating a sum ofdeviations with respect to each pixel pair as in Equation 2. Of course,the pixels of one pixel pair may overlap with those of the other pixelpair.

For example, in FIG. 7A, the pixel 03_03 makes a pair with the pixel02_03 and a pair with the pixel 04_03.

In this manner, the process of calculating the cost of the specificdirection is equally applied as in FIGS. 7B through 7H. However, thepixel pair is formed between the adjacent pixels in the case of FIGS. 7Athrough 7D, whereas the pixel pair may be formed between the pixels thatare not adjacent to each other in the case of FIGS. 7E through 7H. Thisis because, in the case of FIGS. 7E through 7H, a slope is ±2 or ±½. Ofcourse, a method of interpolating the adjacent pixels without using theseparated pixels is possible. For example, it is shown in FIG. 7E thatthe pixel 03_04 makes a pair with the pixel 01_03, but the pixel 03_04may make a pair with half pixels interpolated from the pixel 02_03 andthe pixel 02_04.

Meanwhile, in regard to the eight modes having directionality among thenine intra modes, the cost C_(i) of each directionality can becalculated by forming the pixel pair as in FIGS. 7A through 7H andsubstituting the formed pixel pair into Equation 1. However, since thecost C₂ of mode 2 of the intra modes is calculated by an average withoutdirectionality, D_(ik) (i=2) of Equation 2 can be obtained from Equation3 below.

D _(2k) =|μ−p _(k)|  [Equation 3]

Here, P_(k) is the value of the k-th pixel among the n pixels selectedfrom the neighboring blocks, and μ is the average of the pixel values,i.e. 1/n×ΣP_(k). When D_(2k) obtained in this way is substituted intoEquation 2, the cost C₂ of mode 2 can be finally calculated.

FIG. 8 shows an example of selecting n pixels from neighboring blocks inorder to calculate a cost of mode 2. Since n is set to 12 in theembodiments of FIGS. 7A through 7H, a total of 12 adjacent pixels areselected in FIG. 8 so as to correspond to the set value.

The mode selector 210 or 540 can calculate the costs C_(i) of a total ofnine intra modes as shown in the embodiments of FIGS. 7A through 7H and8, and then select one having a minimum cost from among the intra modes.Since this selected intra mode is obtained without using informationabout the current block, it can be used in common for the video encoder200 and the video decoder 500.

In example embodiments of the present invention, the directionality ofthe neighboring blocks is used to select the intra mode suitable for thecurrent block. This starts on the assumption that the directionalitiesof the small-sized neighboring blocks such as 4×4 blocks are similar toeach other. Of course, when the directionality of the neighboring blockis different from that of the current block, there is a possibility ofthe energy of a residual increasing compared to the existing H.264. Thispossibility is nothing but a problem of trade-off for benefits obtainedby not encoding the intra mode information. However, since the intramode information should be encoded in units of a small size of 4×4, anamount of bits reduced by omitting the encoding is considerable. Assuch, if a difference between the directionalities of the neighboringblock and the current block is not great, the method of exampleembodiments of the present invention can be regarded to be efficient.Further, if a plurality of (typically, four) neighboring blocks ratherthan a single neighboring block are referred to, it is possible tominimize the difference between the directionalities of the neighboringblock and the current block.

FIG. 9 shows the structure of a bitstream 150 according to an exampleembodiment of the present invention. In H.264, such a bitstream isencoded in units of slices. The beat stream 150 may include a sliceheader 160 and slice data 170. The slice data 170 is made up of aplurality of macroblock data (MB) 171 to 174. Further, one 173 of theMBs may be made up of an mb_type field 180, an mb_pred field 185, and atexture data field 189.

Here, the mb_type field 180 is recorded with a value indicating a typeof the macroblock. In other words, the value indicates whether a currentmacroblock is an intra macroblock or an inter macroblock.

Further, the mb_pred field 185 is recorded with a detailed predictionmode based on the macroblock type. In the case of the inter macroblock,information about a reference frame number and a motion vector isrecorded according to each macroblock partition. However, in exampleembodiments of the present invention, in the case of the intramacroblock, the selected intra mode does not need to be recorded in thisfield, unlike the conventional H.264. In other words, in the case of theintra macroblock, the mb_pred field 185 can be omitted.

In the case of the conventional H.264, the mb_pred field 185 includesblock information fields 191 to 194 for recording the intra modeaccording to each block (4×4 block). In the intra macroblock accordingto example embodiments of the present invention, these block informationfields are omitted, so that it is possible to remarkably reduce a sizeof the bitstream.

Like the conventional H.264, the texture data field 189 is recorded withencoded residual signals, i.e. texture data.

According to example embodiments of the present invention, in comparisonwith an existing H.264 standard, since the intra mode information is notcoded while using intra-prediction coding as it stands, it is greatlyreduce the overhead of a bitstream transmitted from an encoder to adecoder.

Thus, when the coded bitstreams are identical in size, it is possible toprovide a higher quality to an image reconstructed at the stage of avideo decoder.

While the invention has been shown and described with reference tocertain example embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. Therefore, it is to be understood thatthe embodiments described herein are illustrative in every respect andnot restrictive.

1. A video encoding apparatus comprising: a mode selector configured toselect one from among a plurality of intra modes on the basis ofdirectionality of at least one neighboring block, the at least oneneighboring block having already been reconstructed before the currentblock is reconstructed; an intra predictor configured to obtain aprediction block of the current block from the at least one neighboringblock according to the directionality corresponding to the selectedmode, and configured to obtain a residual block by subtracting theprediction block from the current block; and means for encoding theobtained residual block.
 2. The video encoding apparatus of claim 1,wherein the encoding means includes: a spatial transformer configured totransform the residual block into a frequency domain to generate atransform coefficient; a quantizer configured to quantize the transformcoefficient; and an entropy encoder configured to perform losslessencoding on the quantized result.
 3. The video encoding apparatus ofclaim 1, wherein the mode selector calculates costs based on arespective plurality of directions in the at least one neighboring blockthat includes at least one of a upper block, a left block, a upper leftblock, and a upper right block with respect to the current block, andselects an intra mode, which corresponds to the direction at which thecalculated cost has a minimum value, from among the plurality of intramodes.
 4. The video encoding apparatus of claim 3, wherein the costsbased on the respective plurality of directions are calculated by a sumof deviations between pixel pairs according to corresponding directionsin the at least one neighboring block.
 5. The video encoding apparatusof claim 4, wherein the costs are calculated by a sum of averagedeviations of values of a plurality of pixels selected from the at leastone neighboring block when an intra mode does not represent a direction,and the pixel pair is selected from pixels most adjacent to the currentblock within the at least one neighboring block.
 6. A video decodingapparatus comprising: means for reconstructing a residual signal of acurrent block from an input bitstream; a mode selector configured toselect one from among a plurality of intra modes on the basis ofdirectionality of at least one neighboring block, the at least oneneighboring block having already been reconstructed before the currentblock is reconstructed; and an intra reconstructor configured to obtaina prediction block of the current block from the at least oneneighboring block according to a directionality of the selected mode,and configured to reconstruct the current block by adding the residualsignal of the current block and the prediction block.
 7. The videodecoding apparatus of claim 6, wherein the reconstructing meansincludes: an entropy decoder configured to perform lossless decoding onthe bitstream to reconstruct a quantization coefficient; a dequantizerconfigured to reconstruct a transform coefficient from the quantizationcoefficient; and an inverse spatial transformer configured to inverselytransform the transform coefficient to reconstruct the residual signalin a spatial domain.
 8. The video decoding apparatus of claim 6, whereinthe mode selector calculates costs based on a respective plurality ofdirections in the at least one neighboring block that includes at leastone of a upper block, a left block, a upper left block, and a upperright block with respect to the current block, and selects an intramode, which corresponds to the direction at which the calculated costhas a minimum value, from among the plurality of intra modes.
 9. Thevideo decoding apparatus of claim 8, wherein the costs based on therespective plurality of directions are calculated by a sum of deviationsbetween pixel pairs according to corresponding directions in the atleast one neighboring block.
 10. The video decoding apparatus of claim9, wherein the costs are calculated by a sum of average deviations ofvalues of a plurality of pixels selected from the at least oneneighboring block when an intra mode does not represent a direction. 11.The video decoding apparatus of claim 9, wherein the pixel pair isselected from pixels most adjacent to the current block within the atleast one neighboring block.
 12. A video decoding method comprising:reconstructing a residual signal of a current block from an inputbitstream; selecting one from among a plurality of intra modes; andperforming an intra-prediction according to a directionality of theselected intra mode to reconstruct the current block.
 13. The videodecoding method of claim 12, wherein the plurality of intra modes use 34modes for an 8×8 block.
 14. The video decoding method of claim 13,wherein the 34 modes have a maximum of 34 directions, each of which isan arbitrary direction with respect to on the basis of an arbitrarypixel within the current block, the arbitrary direction having a slopedefined as dy in a vertical direction over dx in a horizontal direction,wherein dx and dy are natural numbers.
 15. The video decoding method ofclaim 12, wherein the number of the plurality of intra modes isdifferent depending on a block size.
 16. The video decoding method ofclaim 15, wherein the plurality of intra modes include at least one of 9intra modes for a 4×4 block, 9 intra modes for an 8×8 block, 34 intramodes for a 16×16 block, 34 intra modes for a 32×32 block, 5 intra modesfor a 64×64 block, and 5 intra modes for a 128×128 block.
 17. The videodecoding method of claim 12, wherein the selecting of one from among aplurality of intra modes includes selecting one from among the pluralityof intra modes on the basis of a directionality of at least oneneighboring block that has already been reconstructed before the currentblock is reconstructed.
 18. The video decoding method of claim 12,wherein the performing an intra-prediction according to a directionalityof the selected intra mode to reconstruct the current block includes:obtaining a prediction block of the current block from the at least oneneighboring block according to the directionality of the selected mode;and reconstructing the current block by adding the residual signal ofthe current block and the prediction block.
 19. The video decodingmethod of claim 18, wherein the at least one neighboring block includesat least one of a upper block, a left block, a upper left block, and aupper right block with respect to the current block.